The
PIC12F615 was perfect for this project. It runs off a 3 volt lithium cell.

The unit is self calibrating. Just wait until
your plant needs watering, insert the battery, and place in the soil.
This sets the threshold for sensing dry soil.

When the soil dries out to this point, the
unit begins 'chirping' about every 20 minutes.

A CDS cell measures room light preventing
operation at night, and the battery voltage is also monitored. A 'beeping'
sound is made if the battery is low.

The probes are just bare wire soldered to
the lower section of the circuit board. These are covered with heat-shrink
tubing to insulate them from the soil. Non-insulated probes suffer from
a number of problems - this eliminates all of them.

Referring to the drawing above, the PIC generates high
frequency pulses on GP5 driving one of the probes. The signal passes through
the soil via capacitance, but the reading is based on the values of soil
resistance (R/Soil) vs load resistor R1. Voltage spikes from this voltage
divider are accumulated on C1 via CR1. The resulting voltage is read by
AN3. This reads the soil conductivity with no direct connection and avoids
a lot of galvanic complications.